1. Field of the Invention
The present invention relates to a frequency modulator utilizing a magneto-static wave for obtaining a frequency-modulated output high frequency electro-magnetic wave by varying a propagation velocity of the magneto-static wave through the modulation of an applied DC magnetic field, which magneto-static wave is generated in a ferri-magnetic member applied with said DC magnetic field through the mutual conversion from the high frequency electro-magnetic wave, particularly, to a ferri-magnetic film frequency modulator having a good linearity through a small-sized and readily manufactured arrangement of a ferri-magnetic film, so as to be stably operated with a high sensitivity.
2. Related Art Statement
Among conventional frequency modulators of various circuit systems, those which utilize a voltage-controlled oscillator (VCO) and a varactor respectively are typical.
In the frequency modulator utilizing the VCO, as shown in FIG. 1, a part of high frequency output of the VCO1 is supplied to a prescaler for dividing the frequency thereof by 1/n, so as to be phase-compared with an output oscillation of a reference oscillator 4 in a comparator 5, a resultant DC voltage being applied on a resonant circuit containing a varactor C.sub.0 in the VCO1 for fixing the oscillation frequency thereof at the reference, and a modulation signal voltage from a modulator 2 being superposed thereon for the frequency modulation.
However, the conventional frequency modulator arranged as mentioned above has a defect that a low-pass filter 6 is necessitated for applying the resultant DC voltage on the VCO1 and hence it is difficult to attain the frequency modulation effected by the low frequency modulation signal.
On the other hand, in the frequency modulator effecting the phase modulation through a multi-stage filter employing varactors, as shown in FIG. 2A, the phase through the low pass filter (LPF) of n stages consisting of varactors C.sub.1, C.sub.3, C.sub.5, . . . and inductors L.sub.2, L.sub.4, . . . is delayed by n.pi./2 radian. When capacities of those varactors are varied, the cutoff frequency of the LPF is varied. For instance, when those capacities are increased, the phase characteristic curve as shown by a solid line in FIG. 2B is varied as shown by a dotted line, and, as the result, the phase .theta. at the frequency f.sub.o is varied by .DELTA..theta..
Meanwhile, when the signal which is phase-modulated as mentioned above is denoted by V.sub.PM, the signal V.sub.PM can be expressed by the following equation(1). EQU V.sub.PM =Acos{2.pi.f.sub.c t+V.sub.(t) -.theta..sub.o } (1)
And hence, the instant angular frequency of the frequency-modulated signal can be expressed by time-differentiating the portion bracketed in {} of the equation (1) as follows. ##EQU1##
When the following equation (2) is given with regard to the modulation of frequency f.sub.m and the maximum phase delay .theta..sub.max, EQU V.sub.(t) =.theta..sub.max sin(2.pi.f.sub.m t) (2)
the instant angular frequency can be expressed as follows. EQU 2.pi.{f.sub.c +.theta..sub.max f.sub.m cos(2.pi.f.sub.m t)}
Furthermore, the maximum frequency deviation denoted by .DELTA.f can be expressed as follows. ##EQU2##
In this connection, this equation (3) is well-known and is called a "modulation index".
Accordingly, when the modulation frequency f.sub.m and the frequency deviation .DELTA.f are given, the frequency modulation can be effected by varying the voltage applied on the varactors in the LPF as shown in FIG. 2A until the equation .DELTA..theta.=.theta..sub.m is attained.
However, the lower the modulation frequency f.sub.m is lowered, the more the phase variation .DELTA..theta. is increased, and hence it is required the more to vary the voltage applied on the varactors, as is apparent from the equation (3).
Consequently, the conventional frequency modulator arranged as mentioned above has a defect that the nonlinear distortion is resulted from the property of the varactor.
In place of these conventional frequency modulators respectively having the defects as mentioned above, a different frequency modulator in which the frequency modulation is effected by varying the propagation phase of a wave has been conventionally developed.
When a situation such that n waves are propagating successively on a line having a length L is supposed, the wave length .mu. of those waves is expressed by the following equation (4). ##EQU3##
When this wave length .lambda. is varied by .eta..lambda. through any means, the phase variation .DELTA..theta. on the line having the length L can be expressed by the following equation (5). ##EQU4##
In this regard, for providing the frequency modulator arranged as mentioned above in a small size, it is required to reduce the line length L, while it is preferred to reduce the factor .eta. for improving the linearity also.
On the other hand, as described before, the lower the modulation frequency f.sub.m is lowered, the more phase variation .DELTA..theta. is required to be, so that it is required according to the equation (5) that the wave length .lambda. is reduced.
For example, in the case that the maximum frequency deviation .DELTA..sub.f =5 kHz, it is required according to the equation (3), to attain the following phase variation .DELTA..theta.. ##EQU5##
In the case that L=3 mm and .eta.=0.1, the following wave length .lambda. is obtained according to the equation (5). ##EQU6##
As for the wave propagating on the line with the short wavelength as mentioned above, (a) an elastic wave and (b) a magnetic wave can be regarded. However, for the elastic wave (a), it is difficult to vary the wavelength by externally applying an electric field or a magnetic field on the propagation line, so that it is extremely difficult to realize the frequency modulator arranged as mentioned above by employing the elastic wave. Consequently, that which can be expected to be realized is the frequency modulator arranged as mentioned above by employing the magnetic wave (b).
In this connection, the magnetic wave can be generally regarded as the wave which propagates through a ferri-magnetic member applied with a DC magnetic field, and, when the vector of the applied magnetic field is denoted by H, it can be regarded as the wave such that the energy of the electric field, which satisfies the condition as expressed by the following equation (.eta.), is substantially zero. EQU .gradient..times.H.perspectiveto.0 (7)
For instance, the relation between the angular frequency .intg. of the wave which propagates through the ferri-magnetic member having an infinite region and the number k of those waves contained in the unit length 1 cm can be expressed as shown in FIG. 3. In FIG. 3, .theta.indicates an angle between the direction of the applied DC magnetic field and the propagation direction f the wave, while the wave consists of an electro-magnetic wave in the region I, of a magneto-static wave in the region II and of that which is called as an exchanged spin wave in the region III.
Among the waves of these three kinds, with regard to the exchanged spin wave in the region III, the number of waves contained in the unit length is large, and hence the wave length is short. For example, when the wave number k=10.sup.5 in FIG. 3, the wave length .lambda. becomes as follows. ##EQU7##
On the other hand, as described later with regard to the frequency modulator of this kind, the size of the electrodes employed for the conversion between the electro-magnetic wave and the magneto-static wave is required to have a precision of the same order as the wavelength of those waves. So that, in the case that the wavelength .lambda. is too short, it is difficult to realize the frequency modulator of this kind. In this regard, the wavelength of the magneto-static wave in the region II as shown in FIG. 3 is in order of several tens .mu.m, so that a sufficiently short wavelength .lambda. can be obtained in the realizable range. Consequently, the magneto-static wave is the most suitable for obtaining the wavelength such as expressed by the equation (6).
In this connection, FIG. 3 relates to the propagation medium having an infinite region, while in the situation of the finite region, for instance, such that the DC magnetic field is applied in the axial direction of a fine cylindrical ferri-magnetic body, the magneto-static wave propagation in the axial direction is generated, and the phase velocity thereof can be expressed by the following equation (8). ##EQU8## where, R is a radius of the cylindrical ferri-magnetic body: v.sub.o is the velocity of an electro-magnetic wave in vacuum;
.epsilon..sub.r is a specific permittivity of the ferri-magnetic body; and PA1 .mu..sub.+,r is a positive circular-polarized specific permeability, the value of which is abruptly varied at the resonant magnetic field Hr, as shown in FIG. 4.
In this connection, as is apparent from the equation (8), v'.sub.p is the phase velocity of the electro-magnetic wave which propagates through an isotropic medium having the specific permeability .mu..sub.30 and the specific permittivity .epsilon..sub.r, so that the phase velocity v.sub.p of the magneto-static wave is slower than that v'.sub.p of the electro-magnetic wave in proportion to R/.lambda..sub.c, and hence the smaller the radius R of the cylindrical ferri-magnetic body is, the slower the phase velocity of the magneto-static wave is. As a result, the wave length .lambda. of the magneto-static wave becomes shorter according to the following equation (9). ##EQU9##
On the other hand, as is apparent from FIG. 4, when the intensity H.sub.in of the applied DC magnetic field is varied, the specific permeability .vertline..mu..sub.+,r .vertline. is varied and hence the phase velocity v'.sub.p of the electro-magnetic wave is varied and further the phase velocity v.sub.p of the magneto-static wave is varied. As a result, the wave length .lambda.is varied. The sensitivity of this variation of the wave length .lambda., which is caused by the variation of the applied magnetic field intensity H.sub.in, that is, ##EQU10## is abruptly increased in the vicinity of the resonant magnetic field intensity H.sub.r in which the variation of the specific permeability .vertline..mu..sub.+ .vertline. is significant. Accordingly, for increasing the sensitivity of the aforesaid variation of the wavelength, it is required to utilize the vicinity of the resonant magnetic field in which H.sub.in .perspectiveto.H.sub.r is attained. However, in the vicinity of the resonant magnetic field, even when the applied magnetic field intensity H.sub.in is slightly varied for any reason, as is apparent from the above, the wavelength .lambda. is varied significantly and hence the instability is caused.
However, when the applied magnetic field intensity H.sub.in is kept apart from the resonant magnetic field intensity H.sub.r as far as possible for avoiding the above mentioned unstable operation of the magneto-static wave, the sensitivity .differential..vertline..mu..sub.+ .vertline./.differential.H.sub.in of the variation of the specific permeability is reduced and hence the sensitivity .differential..lambda./.differential.H.sub.in of the variation of the wavelength is reduced, and, as a result, the sensitivity .differential..theta./.differential.H.sub.in of the overall phase variation is reduced also. Consequently, it is required to shorten the wavelength .lambda. of the magneto-static wave and hence to simultaneously place as many waves as possible on the cylindrical ferri-magnetic body having the length L, so that, as is apparent from the equations (8) and (9), it is required to reduce the radius of the cylindrical ferri-magnetic body as fine as possible.
However, in the case that the radius of the cylindrical ferri-magnetic body is reduced, it is extremely difficult to manufacture it and further the structure thereof becomes fragile. With regard to the frequency modulator employing the cylindrical ferri-magnetic body, which is described in Japanese Patent Publication No. 47-22,050 by the present inventor, these defects on manufacture and practical use have been actually observed. Consequently, the removal of these defects has been conventionally regarded as the task to be accomplished.